Polysaccharide fouling in reverse osmosis and forward osmosis desalination and its alleviation

Membrane separation processes, including forward osmosis (FO) and reverse osmosis (RO), for application in water desalination are plagued by membrane fouling. In particular, membrane biofouling is unpredictable in its nature and affected by numerous factors. One of the major contributors to biofouling is the extracellular polymeric substances (EPS) produced by bacteria, especially the polysaccharides that form a large part of EPS. The objectives of this study are to understand the polysaccharide fouling mechanisms based on a comparison of polysaccharide fouling in FO and RO and to find suitable alleviating agents for polysaccharide fouling mitigation. Three major tasks were conducted in this study. Firstly, polysaccharide fouling in FO and RO were compared under identical solution chemistry and operational conditions to understand the respective fouling mechanisms in FO and RO. Secondly, some alleviating agents for mitigation of polysaccharide fouling in FO and RO were tested to demonstrate the fouling alleviation mechanism. Thirdly, a model of hydraulic resistances was developed to illustrate membrane fouling mechanisms based on analysis of the contribution of hydraulic resistances to permeate flux decline. Major findings are: 1) Commercial polysaccharides and polysaccharides isolated from naturally adherent bacteria behaved differently in membrane fouling, which showed that alginate was not a typical model and it is important to select a proper model for polysaccharide fouling. 2) Under identical conditions, membrane fouling by both commercial and isolated polysaccharides was more severe in RO than FO, indicating the importance of pressure source in membrane fouling. 3) RO fouling was likely dominated by foulant – foulant interaction which was greatly affected by calcium ions, while FO fouling could be largely governed by foulant – membrane interaction, which was greatly influenced by solution viscosity. 4) Sodium nitroprusside (SNP) at a proper dose was found to be able to reduce membrane fouling, which could be explained by the electrostatic repulsion between polysaccharides and SNP. 5)Presence of calcium ions played a crucial role in polysaccharide fouling and its alleviation, with its presence leading to significant increase in cake resistance in RO fouling and reducing alleviation efficiency